Protestors who believe in small-d democracy may sue the big-D Democrats over their crackdown on dissent at the convention: Just six weeks before the Democratic National Convention, civil rights groups are threatening to sue the city of Boston because a "free-speech zone" near the FleetCenter remains piled high with twisted steel and hunks of Big Dig concrete, no protest groups have received demonstration permits, and the city is enforcing its rule against afternoon marches. The threats of legal action followed a meeting yesterday between the city and the American Civil Liberties Union and National Lawyers Guild, where activists were told of a Boston ordinance that bans marches from 3:30-6:30 p.m. Those are hours when evening commuter traffic is heaviest. But the civil rights groups complain that the rule effectively bars them from parading in streets when delegates will be arriving at the FleetCenter for convention activities. The civil rights groups were also told demonstrators may not be allowed to carry signs on sticks for security reasons, and that the use of battery-powered bullhorns will require special permits. Those rules don't appear on the city's website guide to demonstration permitting. "Here we are, six weeks before the convention," said Jeffery Feuer, a National Lawyers Guild attorney who took part in yesterday's meeting. "The DNC has been planned for more than two years. We've been meeting with them since last July, so it's not acceptable anymore. Unless we hear imminently that the procedures have been changed and the constitutional issues are addressed and people can take part in the process, then we're going to file a lawsuit." Merita A. Hopkins, corporation counsel for Mayor Thomas M. Menino, said that some of the civil rights groups' complaints appear to have merit, including one that permits are not being issued quickly enough.